The present invention relates to a liquid ejection head for ejecting droplets by exerting electrostatic forces on a solution in which charged particles are dispersed, and an image recording apparatus including the liquid ejection head. In particular, the present invention relates to a liquid ejection head capable of maintaining a meniscus at a high position, and an image recording apparatus including the liquid ejection head.
Known examples of liquid ejection heads for ink jet that perform image recording (drawing) by ejecting ink droplets include an ejection head for so-called thermal ink jet that ejects ink droplets by means of expansive forces of air bubbles generated in ink through heating of the ink, and an ejection head for so-called piezoelectric-type ink jet that ejects ink droplets by giving pressures to ink using piezoelectric elements.
In the case of the thermal ink jet, however, the ink is partially heated to 300° C. or higher, so there arises a problem in that a material of the ink is limited. On the other hand, in the case of the piezoelectric-type ink jet, there occurs a problem in that a complicated structure is used and an increase in cost is inevitable.
Known as ink jet that solves the problems described above is electrostatic ink jet which uses ink containing charged colorant particles (fine particles), exerts electrostatic forces on the ink, and ejects ink droplets by means of the electrostatic forces (for example, JP 10-230608 A).
An ejection head for the electrostatic ink jet includes an insulating ejection substrate in which many through holes (ejection ports) for ejecting ink droplets are formed, and ejection electrodes that respectively correspond to the ejection ports, and ejects ink droplets by exerting electrostatic forces on ink through application of predetermined voltages to the ejection electrodes. More specifically, with the construction, the ejection head ejects the ink droplets by controlling on/off of the voltage application to the ejection electrodes (driving the ejection electrodes by modulation) in accordance with image data, thereby recording an image corresponding to the image data onto a recording medium.
An example of such a liquid ejection head for the electrostatic ink jet is disclosed in JP 10-230608 A as an ejection head 200. As conceptually shown in FIG. 7, the ejection head 200 includes a support substrate 202, an ink guide 204, an ejection substrate 206, an ejection electrode 208, a bias voltage source 212, and a drive voltage source 214.
In the ejection head 200, the support substrate 202 and the ejection substrate 206 are each an insulating substrate and are arranged to be spaced apart from each other by a predetermined distance.
Many through holes (substrate through holes) that each serve as an ejection port 218 for ink droplets are formed in the ejection substrate 206, and a gap between the support substrate 202 and the ejection substrate 206 serves as an ink flow path 218 for supplying ink Q to the ejection port 218. In addition, the ring-shaped ejection electrode 208 is provided to the upper surface of the ejection substrate 206 (the surface of the ejection substrate 206 on the side from which ink droplets R are ejected) to surround the ejection port 218. The bias voltage source 212 and the drive voltage source 214 serving as a pulse voltage source are connected to the ejection electrode 208, which is grounded through the voltage sources 212 and 214.
On the other hand, the ink guide 204 is provided to the support substrate 202 so as to correspond to each ejection port 218. The ink guide 204 extendes through the ejection port 218 and protrudes from the ejection substrate 206. Also, an ink guide groove 220 for supplying the ink Q to a tip end portion 204a of the ink guide 204 is formed by cutting out the tip end portion 204a by a predetermined width.
In an (ink jet) recording apparatus disclosed in JP 10-230608 A using the ejection head 200 described above, at the time of image recording, a recording medium P is supported by a counter electrode 210.
The counter electrode 210 functions not only as a counter electrode for the ejection electrode 208 but also as a platen for supporting the recording medium P at the time of the image recording, and is arranged to face the upper surface of the ejection substrate 206 and to be spaced apart from the tip end portion 204a of the ink guide 204 by a predetermined distance.
In the ejection head 200, at the time of the image recording, an ink circulation mechanism (not shown) causes the ink Q containing the charged colorant particles to flow in the ink flow path 216 in a direction, for instance, from the right side to the left side in FIG. 7. Note that the colorant particles of the ink Q are charged to the same polarity as the voltage applied to the ejection electrode 208.
The recording medium P is supported by the counter electrode 210 and faces the ejection substrate 206.
Further, a DC voltage of, for example, 1.5 kV is constantly applied from the bias voltage source 212 to the ejection electrode 208 as a bias voltage.
As a result of the ink Q circulation and the bias voltage application, by the action of surface tension of the ink Q, a capillary phenomenon, an electrostatic force due to the bias voltage, and the like, the ink Q is supplied from the ink guide groove 220 to the tip end portion 204a of the ink guide 204, a meniscus M of the ink Q is formed at the ejection port 218, the colorant particles move to the vicinity of the ejection port 218 (migration due to an electrostatic force), and the ink Q is concentrated in the ejection port 218 or the tip end portion 204a. 
In this state, when the drive voltage source 214 applies a pulse-shaped drive voltage of, for example, 500 V corresponding to image data (drive signal) to the ejection electrode 208, the drive voltage is superimposed on the bias voltage and the supply and concentration of the ink Q to the tip end portion 204a are promoted. When a movement force of the ink Q and the colorant particles to the tip end portion 204a and an attraction force from the counter electrode 210 exceed the surface tension of the ink Q, a droplet (ink droplet R) of the ink Q in which the colorant particles are concentrated is ejected.
The ejected ink droplet R flies due to momentum at the time of the ejection and the attraction force by the counter electrode 210, adheres to the recording medium P, and forms an image.
As described above, the liquid ejection head for electrostatic ink jet ejects the ink droplets R by controlling a balance between the surface tension of the ink Q and the electrostatic force exerted on the ink Q.
Accordingly, in order to perform the ejection of the ink droplets at a low drive voltage as well as high speed (high recording (ejection) frequency) with stability, the ink guide provided for each ejection port is important and is required to suitably guide the ink and appropriately stabilize the meniscus of the ink at the ejection port (hereinafter referred to as a “meniscus stability”), capability (hereinafter referred to as a “electric field concentrating capability”) to favorably concentrate the electrostatic force, and the like.
In order to achieve such characteristics, in the liquid ejection head for electrostatic ink jet, the ink guide is devised in various manners.
For instance, in the liquid ejection head disclosed in JP 10-230608 A, as described above, by notching the tip end portion 204a of the ink guide 204 by a predetermined width to form the ink guide groove 220, capability of supplying the ink Q to the tip end portion 204a of the ink guide 204 is further improved.
In order to obtain an ink guide for holding a favorable meniscus with stability in the manner described above, it is preferable that the ink guide be molded with favorable moldability and with high precision so that ink can be guided with reliability.
In order to convey colorant particles up to a guide tip end portion, it is required to form a favorable meniscus so that the tip end portion is wetted with the ink solution. A pressure required for the liquid to wet up to the tip end portion having a shape with a pointed tip end is inversely proportional to the radius of curvature of the tip end, as expressed by the formula (1) given below.P=2·γ/R  (1)
In the above formula (1), P is a pressure (Pa) required to hold the meniscus, γ is the surface tension (N/m) of the liquid forming the meniscus, and R is the radius of curvature (m) of the meniscus.
It can be seen, from the above formula (1), that as the radius of curvature or the thickness of the tip end portion of the ink guide is reduced, the pressure to form the meniscus is required to be increased.
However, there is a limitation on the pressure that can be given to increase the height of the meniscus. Therefore, in the case of the ink guide 204 disclosed in JP 10-230608 A, the ink guide groove 220 is formed in the tip end portion 204a and the meniscus M is held at a high position by utilizing capillary action by the ink guide groove 220.
In a structure of the ink guide 204 disclosed in JP 10-230608 A, however, the tip end portion 204a is notched, so there is a problem in that the sharpness of the tip end portion 204a is lowered and the size of the ink droplet that can be ejected is limited.
Also, in the structure of the ink guide 204 disclosed in JP 10-230608 A, the tip end portion 204a is notched, so the tip end shape of the ink guide 204 is formed by the ink Q. Therefore, the tip end shape of the ink guide 204 is determined by the surface tension of the ink Q used and the pressure exerted on the ink Q. The tip end shape formed by the ink Q fluctuates due to disturbance such as vibration or supply of the ink Q for replenishment of the ink Q consumed through ejection of the ink droplets R. Therefore, there is a problem in that ink adhering position accuracy is lowered, so that it is almost impossible to form an image with stability and at high resolution.
Further, there is a problem in that it is difficult to reduce the width of the tip end portion of the ink guide from the viewpoint of processing. Still further, the ink guide 204 disclosed in JP 10-230608 A requires to form the ink guide groove 220 therein, so processing becomes particularly difficult when the width of the tip end portion is reduced.